Bio-char is the dry component that results when biomass is heated to high temperatures in the substantial absence of oxygen. Bio-char is typically produced as chunk-like, irregularly shaped particles that resemble pieces of lump charcoal. These pieces have high porosity, with multitudes of pores extending into the char from its surface.
There are a number of techniques for producing bio-char, one common one being pyrolysis, in which biomass is heated in the substantial absence of oxygen until polymers of the biomass fall apart. A common approach involves lighting biomass feedstock on fire, then at some point closing the fire off from air. The fire consumes the remainder of the oxygen, and the residual heat converts the biomass to bio-char. For typical polymers such as cellulose, this begins to happen at a temperature of approximately 330° C. In addition to carbonizing the biomass, these high temperatures generate gases within the biomass. The rapid expansion of these gases forms pores within the bio-char. For example, at moderate heating rates and temperatures of approximately 400-500° C., bio-char is produced with a significant number of pores that are typically long but narrow, e.g., about 0.7-3.0 microns in diameter. This high porosity creates high particle surface area (i.e., the total surface area including that of the pores, by weight of the bio-char). For instance, bio-char can commonly have particle surface areas in the range of 50-300 m2/g. However, even with this high porosity, the small pore width allows for low water penetration into the bio-char.
Bio-char can be produced in both batch process systems and continuous process systems. Batch process systems are well known and involve processing batches of biomass in an airtight oven. Continuous process systems make/output biomass on a continuous basis, rather than in batches, and various configurations of such systems can be employed. One common system involves a fluidized bed reactor, such as that described in U.S. Pat. No. 4,064,018, which is hereby incorporated by reference. Another system employs an externally-fired rotary kiln, and is described in U.S. Pat. No. 4,300,915, which is also hereby incorporated by reference.
Many recent efforts have focused on utilizing the above-described porosity, as well as other characteristics of bio-char, in agricultural applications. In particular, there are two distinguishing factors that make bio-char valuable as a soil conditioner. First, bio-char is long-lasting, with some estimating that upwards of 80% of bio-char applied to soil will remain 100 years after application, depending on factors such as the feedstock used to produce the bio-char. Second, as above, the char is very porous, with many narrow pores that allow the char to retain beneficial compounds while resisting water leaching. That is, the farther within the pores a compound can be deposited, the longer it will take to be transported out into soil. It is therefore desirable to embed compounds or agents relatively deep within the pores of bio-char, where low water penetration allows for long, slow release of the compounds. However, the small pore size makes such deep embedding of agents challenging.